To answer your primary question: "Deem" is a stative verb. It represents an opinion of someone at some particular point in time. Although that opinion may continue to be valid, we do not say that someone continues to deem [somebody] to be [some characteristic]. You "deem" once, and then you are done "deeming".
That being said, I don't see a reason why this should matter whether you use "when" or "after". Either may be used:
When/After he cracked the code, experts deemed him to be a genius
The main problem with your sentence is that it is confusing. It seems you wants some achievement to be the subject, but you write the sentence with the person himself as the subject. You have to match the subject to the characteristic.
I would instead start with a gerund phrase, and avoid the when/after issue entirely:
Cracking the Trident code was [quickly, right away, immediately] deemed by many as the major achievement of his career.
Otherwise, if he himself is the subject, then he has to be deemed some characteristic that can be ascribed to a person.
Soon after he cracked the code, he was deemed by many to be the preeminent expert on decryption techniques.
Avoid unnecessary adverbs. Unless it's really important to mention how rapidly he was deemed", there is no need to include "quickly" or "immediately" in your writing, especially in the middle of the verb. If you must include it, put it at the start of the sentence (as in my use of "soon" above).
I'm not sure if "deciphering" is the right term to describe a career, or rather, the general field already has the name cryptology. Someone who works in cryptology ciphers/deciphers, or encrypts/decrypts, on a regular basis.